The present invention relates to the production of hydrotalcites. Hydrotalcites are metal hydroxides having a layer lattice and belonging to the group of anionic clay minerals. Furthermore, the present invention relates to the metal oxides obtained by calcination of the metal hydroxides produced according to this invention.
Metal hydroxides are important precursors for the production of metal oxides used for instance as raw materials for refractories, ceramics, and supports for heterogeneous catalysts. In nature, metal hydroxides predominantly occur in the form of mixed metal hydroxides. There are numerous clay minerals that can be characterised by their layer lattice. The great majority of clay minerals are cationic ones. In said metal hydroxides, cations, e.g. Na+, Ca2+, etc., are located between the negatively charged layers. In anionic clay minerals which are far less common, anions are located between the positively charged layers of the metal hydroxide. A large number of said anionic clay minerals are hydroxides of metals of the main group, namely magnesium and aluminium, and hydroxides of transition metals, such as nickel, chromium, zinc, etc. The structure of said clay minerals can be derived from the brucite structure of magnesium hydroxide Mg(OH)2. In this structure, some of the divalent Mg(OH)64xe2x88x92 octahedra are replaced by Al(OH)63xe2x88x92 octahedra. Examples of said minerals are meixnerite having the idealised unit cell formula Mg6Al2(OH)18.4 H2O and hydrotalcite (Mg6Al2(OH)16CO3.4 H2O). According to the prior art, the magnesium : aluminium atomic ratios can be varied between 1.7 and 4.0. The metal hydroxide octahedra share adjacent edges to form layers. In addition to water, interstitial anions required for balancing the charge are located between the layers. The anion nature can be simple, e.g. OH31 , CO32xe2x88x92, Cl31  or SO42xe2x88x92, or complex, as for instance in large, organic or inorganic anions. Up to now, such anions have been incorporated into the layers by substitution of simple anions or by acid treatment in the presence of the desired anions.
Numerous processes for producing stratiform, anionic clay minerals are known in the art. All of these processes employ metal salts as starting materials which are dissolved and then mixed with each other at defined pH-values. See e.g. U.S. Pat. Specification No. 4,539,306 describing the production of hydrotalcites for pharmaceutical use, and Reichle, W. T., Journal of Catalysis, Vol. 94 (1985), p. 547-557, and Nunan, J. G., et al., Inorganic Chemistry, Vol. 28 (1989), p. 3868-3874. Misra et al. have disclosed the production of hydrotalcites having interstitial anions which increase the interlayer spacing by exchanging the anions at elevated temperatures (cf. U.S. Pat. Specification No. 5,075,089). Examples of the incorporation of large, organic anions by anion exchange are given by Lagaly, G., et al. in Inorganic Chemistry, Vol. 29 (1990), p. 5201-5207. Miyata et al. have described the production of magnesium/aluminium hydrotalcites by mixing solutions of the salts MgCl2 and Al2(SO4)3 and a NaOH solution (cf. Clay and Minerals, Vol. 25 (1977), p. 14-18). EP-A1-0 536 879 proposes the production of stratiform, anionic clay minerals by using inorganic anions which increase the interlayer spacing, such as B(OH)4xe2x88x92, V4O124xe2x88x92, V2O74xe2x88x92, or HV2O73xe2x88x92. In said publication, too, solutions of metal salts are mixed at defined pH-values with solutions of the salts that are to be incorporated. Examples of the uses of stratiform, anionic clay minerals as catalysts are given in U.S. Pat. Specification No. 4,774,212, U.S. Pat. Specification No. 4,843,168, EP-A1-0 536 879, and by Drezdon, M. in ACS Symp. Ser., (Novel Mater. Heterog. Catal.), Vol. 437 (1990), p. 140-148.
WO-A-93 21 961 describes a process for manufacturing of stratiform, mixed metal hydroxides through controlled hydrolysis of metal oxides in a water free organic solvent with stoichiometric amounts of water. The thereby obtained metal hydroxides are gel-compositions for use as a dye carrier in dye laser applications. According to the bottom of page 4 this metal hydroxides have the following composition
MmDdT(OH)4(5xe2x88x92m+d)*nH2O
wherein M, D, T are monovalent, divalent or trivalent metals, m=0 to 1, d=0 to 1 whereby m+dxe2x89xa00. The hereby obtained metal hydroxides are no chemical compounds, but gel-like compositions which are unsuitable as precursors for preparing metal oxides of defined structure according to the scope of the present invention.
The wide use of stratiform, anionic clay minerals has been impeded up to now by the fact that for the production starting from metal salt solutions only a time-consuming, discontinuous synthesis route is known.
Furthermore, catalyst purity is a generally accepted, essential criterion. Contaminations caused by alkalixe2x80x94and alkaline earth metals are particularly undesirable. However, when using metal salts, said contaminants cannot be avoided, or they can only be avoided by great efforts involving high costs. Moreover, there is no process known for producing clay minerals of the hydrotalcite type with only OH31 -ions located in the layers without additional, subsequent ion exchange.
The most important criterion for the catalytic properties of said clay minerals is their basicity. According to the prior art, the basicity is substantially determined by the Mg:Al ratio. See McKenzie, A. L., Fishel, C. T., and Davis, R. J. in J. Catal., Vol. 138 (1992), p. 547. Therefore, in order to adjust the catalytic characteristics of a catalyst, it is desirable to provide the widest possible variation of the Mg:Al ratio. Furthermore, it has been unknown up to now to produce stratiform, anionic clay minerals with a Mg:Al ratio of less than 1.7.
It is the object of the present invention to provide a process for producing stratiform, anionic clay minerals having the following advantages:
time-saving synthesis which can be carried out both continuously and discontinuously
use of inexpensive and readily available starting materials
high purities and low alkali concentrations of the stratiform, anionic clay minerals produced by said process
optionally, the possibility of producing stratiform, anionic clay minerals comprising only hydroxide ions as interstitial anions
production of stratiform, anionic clay minerals having sufficiently large pore volumes and surface areas required for catalysis
production of stratiform, anionic clay minerals having a Mg:Al ratio of less than 1.7.
According to the present invention there is provided a process for producing high-purity hydrotalcites which are stratiform, anionic, mixed metal hydroxides of the general formula
M2x2+M23+(OH)4x+4.A2/nnxe2x88x92.z H2O
wherein x ranges from 0.5 to 10 in intervals of 0.5, A is an interstitial anion, n is the charge of said interstitial anion which is up to 8, normally up to 4, and z is an integer of 1 to 6, particularly 2 to 4, wherein
(A) metal alcoholate mixtures comprising at least one or more divalent metal(s), at least one or more trivalent metal(s), and mono-, di-, or trihydric C1 to C40 alcoholates are used, said means di- and trihydric metal alcoholates being substantially used in a molar ratio corresponding to the stoichiometry of any desired compound according to the empirical formula referred to hereinabove, and
(B) the resultant alcoholate mixture is hydrolysed with water, the water for hydrolysis being used in stoichiometric excess, referring to the reactive valences of the metals used.
The corresponding mixed metal oxide can be produced therefrom by calcination.
The metal alcoholates can be produced by reacting metals having the oxidation numbers +II or +III with mono-, di- or trihydric C1 to C40 alcohols. The production of the metal alcoholates can be accomplished by
(A) placing the metals jointly into the reaction vessel and then adding the alcohol, or
(B) producing the metal alcoholates separately, the alcoholates optionally having different alcoholate residues, or
(C) consecutively, i.e. by placing one metal into the reaction vessel, adding the alcohol, followed by addition of the second metal, and, optionally, of further amounts of alcohol.
Divalent metals suitable for the production of said alcoholates are Mg, Zn, Cu, Ni, Co, Mn, Ca and/or Fe. Suitable trivalent metals are Al, Fe, Co, Mn, La, Ce and/or Cr.
Prior to or during hydrolysis, any water-soluble, di- or trivalent metals can be added as metal salts, the metal salts being used in smaller stoichiometric quantities than the metal alcoholates.
The metal alcoholates are produced such that the molar ratio of divalent : trivalent metal alcoholates is from 1:2 to 10:1. They are subsequently hydrolysed. Prior to hydrolysis, the alcoholate (alcoholate mixture) may be filtered to separate any insoluble component.
Suitable alcohols are mono-, di-, and trihydric alcohols having chain lengths of C1 to C40. They can be branched, unbranched, or cyclic, but branched and unbranched alcohols with chain lengths of C4 to C20 are preferred, and chain lengths of C6 to C14 are particularly preferred.
For the production of stratiform, anionic clay minerals according to this invention, the metal alkoxides may be produced from the same alcohols or mixtures of alcohols.
For the production of high-purity clay minerals, the water used for hydrolysis is purified by ion exchange or repeated distillation. Hydroxide anions and/or any other water-soluble anions can be added to the water for hydrolysis. As organic anions, alcoholate anions are particularly preferred, but alkyl ether sulfates, aryl ether sulfates and/or glycol ether sulfates are also suitable; and/or inorganic anions can be used, particularly carbonate, hydrogen carbonate, nitrate, chloride, sulfate, B(OH)4xe2x88x92; and/or polyoxometal anions, such as Mo7O246xe2x88x92 or V10O286xe2x88x92, are also suitable. NH4+ is the preferred gegenion. The anions are incorporated as interstitial anions into the lattices of the stratiform clay minerals formed during hydrolysis, or they are incorporated subsequently by anion exchange as interstitial anions into the stratiform, anionic clay minerals.
The pH-value of the water for hydrolysis may be in the range of 0 to 14, preferably 1 to 13. The temperature of the water for hydrolysis may be from 5 to 98xc2x0 C., preferably 20 to 95xc2x0 C., most preferably 30 to 90xc2x0 C.
The hydrotalcites produced according to this invention have interlayer spacings (d-values) of greater than 7 xc3x85, measured on the d(003) reflex. The compositions and physical data of Mg(Zn)/Al clay minerals produced according to the invention are listed in Table I.
Metal hydroxides are important precursors for the production of metal oxides. The metal oxides produced according to the invention are used as high-purity raw materials for the production of refractories, ceramics, and catalyst supports. The metal hydroxides can be used as highpurity, inorganic ion-exchange materials and mole sieves, as anticarious additives for tooth pastes, or as antacids, and as additives for plastics, e.g. flame retardants and yellowing inhibitors for PVC.
The stratiform, anionic clay minerals are produced in high purities. This is achieved by the process of the invention which comprises reacting the metals with alcohols yielding alcoholates, followed by purification of the alcoholates, e.g. by filtration. Table II gives a survey of the analytical data of several compounds produced according to the invention, of the starting metals used and of reference products obtained by the conversion of metal salts. Reference product A (RP-A) was produced from metal salts, namely reagent-grade nitrate salts, as reported in literature. Reference product B (RP-B) was produced by conversion of metal hydroxides.
Magnesium powder or granules and aluminium needles (see Table II) were used for producing the compounds of the invention. The data listed in Table II confirm that the compounds produced according to the invention have the desired high purities required for numerous applications. A particular advantage is the significantly lower content of alkalixe2x80x94and alkaline earth metals (sodium and calcium) and of silicon and iron all of which have undesirable effects in catalysis.
The X-ray diffraction patterns depictd in FIG. 1 and FIG. 2 are typical of the compounds produced according to the invention. For comparison, FIG. 3 shows the x-ray diffraction pattern of a compound produced from an aluminium hydroxide/magnesium hydroxide solution. Aluminium hydroxide and magnesium hydroxide are present in unchanged quantities; no clay minerals were formed.
Mixed metal oxides can be produced by calcination of the compounds produced according to this invention. For calcination, the compounds of the invention were placed into a muffle furnace heated to 550xc2x0 C.-1,500xc2x0 C. at which temperatures they were kept for 3 to 24 hours. The mixed metal oxides thus produced had the same high purities as the mixed metal hydroxides of the invention.
The surface areas of calcined compounds obtained at different calcination temperatures are presented in Table III. In order to demonstrate the great surface stability of the compounds produced according to the invention in comparison with a product obtained by mixing metal hydroxides, reference product B (RP-B) was calcined under the same conditions. The metals ratio in reference product B is the same as in Mg6Al2 (OH)18.4H2O produced according to the invention.
Table IV shows a list of metal hydroxides produced according to the invention and the metal oxides produced therefrom by calcination.
The x-ray diffraction pattern of a spinel obtained from compound 1 in Table I is depicted in FIG. 4. For comparison, the dashed line shows the x-ray diffraction pattern recorded in the JCPDS file (entry No. 21-1152, MgAl2O4, spinel, syn). Thus, it has been proved that a pure-phase spinel, MgAl2O4, can be produced by calcining compound 1 according to this invention.